1. Field of the Invention
This invention relates to a device for compensating for the shift of an image plane of a focal length variable lens system when the focal length is changed.
2. Description of the Prior Art
In general, with the focal length variable lens systems or zoom lens systems, when the magnification varying system or variator is moved by zooming to change the focal length, the sharp focus position is shifted. Therefore, despite the above-described optical system which has been adjusted to an in-focus condition before zooming, the sharp focus position is deviated from the in-focus condition by the zooming operation. For this reason, in the past, the compensating system for compensating for deviation of the above-described sharp focus position is operatively connected to the above-described variator by mechanical or electrical means so that the sharp focus position is constantly protected from zooming, as for example, disclosed in U.S. Pat. No. 4,161,756. However, this mechanical or electrical operating means for the variator and compensator requires great precision and accuracy. In order to fully satisfy this, the operating means would become very complex. This has been a great hindrance in reducing the size and weight of the focal length variable lens apparatus and in lowering the production cost.
To solve this, an automatic focus position detector for optically detecting the deviated state of the in-focus position may be used, and by the signal obtained therefrom, the compensator is moved to an electrically adjusted position with the help of an electric motor used as the driving source so that the image shift is compensated for, as proposed in U.S. Pat. No. 3,972,056. In this case, there is no need to use the mechanical or electrical interconnection between the variator and the compensator, thereby producing a simple structure, which is, of course, advantageous. But the above-described automatic focus position detecting device generally has detectable range limits of image shifts. A drawback was that the image shifted too far from the focal plane, and consequently was undetected. For this reason, when applied to such zoom lens systems which allow the image plane to shift from the focal plane by the movement of the variator when zooming, that is, which have long focal lengths, or such zoom lens systems which have high zoom ratios and are capable of closeup photography, there were problems of the image position falling outside the dynamic detection range of the automatic focus position detecting device so that compensation for the image shift became impossible and the response speed became slower.
With the foregoing situations in mind, the present invention has been proposed, its first object being the provision of a focal length variable lens apparatus in which upon movement of the magnification varying system, the image position shifts and the shift is compensated for in such a way that the position of the compensating system is first roughly adjusted in accordance with the moved position of the magnification varying system. After that, by a signal from a focus position detecting circuit, the position of the compensating system is accurately adjusted. An advantage is that even when applied to a focal length variable lens system which has a wide range of image shifts as the magnification varying system moves, without the necessity of employing any complicated mechanical or electrical interconnections between the varying system and the compensating system, it is always possible to accurately adjust the axial position of the compensating system to compensate for the image shift resulting from the movement of the magnification varying system.
Other objects of the invention will become apparent from the following description of the embodiments thereof.